1. Field of the Invention
The present invention relates to fusing devices of image forming apparatus such as a laser beam printer, a multi-function machine, a photocopier, and the like, and more particularly, to a method of temperature control in a fusing device of an image forming apparatus by optimizing a power supply to a heater that heats a fusing roller of the fusing device and minimizing fluctuation in surface temperature of the fusing roller.
2. Description of the Related Art
Generally, an image forming apparatus such as a photocopier, a laser beam printer and the like, prints a desired image on a recording medium such as, a sheet of printing paper, by a series of image forming processes. The image forming processes include electrically charging a surface of a photosensitive drum by rotating an electrostatic charging roller disposed adjacent to the photosensitive drum, exposing the surface of the photosensitive drum to a laser beam projected from a laser scanning unit (LSU) to form an electrostatic latent image on the surface of the photosensitive drum, developing the electrostatic latent image formed on the surface of the photosensitive drum to a toner image of a powdery state (i.e. a powdery visible image) by supplying a toner to the surface of the photosensitive drum, transferring the toner image formed on the surface of the photosensitive drum onto the sheet of printing paper by passing the sheet between the photosensitive drum and a transfer roller which are in contact with each other under a pressure, by supplying a transfer voltage to the transfer roller and the photosensitive drum, and fusing the powdery toner image onto the sheet of printing paper by heating the sheet of printing paper with the toner image transferred thereon through a fusing device including a fusing roller.
Generally, in the process of fusing the toner image, a halogen lamp is employed as a heating source of the fusing device. The halogen lamp is disposed inside the fusing roller and/or a fusing backup roller to radiantly heat a surface of the fusing roller and/or the fusing backup roller to a temperature.
FIG. 1 schematically shows a conventional fusing device 10 of a conventional electrophotographic image forming apparatus.
The fusing device 10 includes a cylinder-shaped fusing roller 11 that has a non-stick coating layer of, for example, TEFLON® on a surface thereon, and a halogen lamp 12 disposed in an inner center thereof. The arrangement of the fusing roller 11 and the halogen lamp 12 is also shown in FIG. 2. The halogen lamp 12 generates heat inside of the fusing roller 11, and the fusing roller 11 is thereby heated by a radiant heat from the halogen lamp 12.
Returning to FIG. 1, disposed under the fusing roller 11 is a fusing backup roller 13. As shown in FIG. 3, the fusing backup roller 13 is resiliently supported by a spring 13a, so as to press a sheet of printing paper 14 passing between the fusing roller 11 and the fusing-backup roller 13, toward the fusing roller 11 with a pressure.
Accordingly, while the sheet of printing paper 14 is passing between the fusing roller 11 and the fusing backup roller 13, a toner image 14a formed on the sheet of printing paper 14 in the powdery state is subjected to a pressure and a heat. As a result, the toner image 14a is fused and fixed onto the sheet of printing paper 14 by the pressure and the heat imposed thereto through the fusing roller 11 and the fusing-backup roller 13.
Returning to FIG. 1, installed at a side of the fusing roller 11 are a thermistor 15, a thermostat 16 and a power switching part 19 such as a thyristor. The thermistor 15 is for detecting or sensing a surface temperature of the fusing roller 11 in an electric signal form, the thermostat 16 is for interrupting an electric power supply to the halogen lamp 12 when the surface temperature of the fusing roller 11 exceeds a given threshold, and the power switching part 19 is for switching a power supply of an AC power source 18 to the halogen lamp 12 according to a signal from a controller 20.
The thermistor 15 senses the surface temperature of the fusing roller 11, and transmits the sensed temperature to the controller 20. The controller 20 compares the sensed temperature with a set temperature, and regulates the power supply to the halogen lamp 12 through the power switching part 19, thereby maintaining the surface temperature of the fusing roller 11 in a print temperature suitable to fuse the toner image 14a and to fix it onto the sheet of printing paper 14.
The controller 20 usually controls the surface temperature of the fusing roller 11 by a temperature control process that includes an initial heating step of heating the surface temperature of the fusing roller 11 to a print standby temperature, a print standby step of maintaining the surface temperature of the fusing roller 11 at the print standby temperature and waiting a print command, and a printing step of maintaining the surface temperature of the fusing roller 11 higher than the print standby temperature to offset a loss in heat during the fusing operation of fusing the toner image.
At each of the operations of the temperature control process, the controller 20 controls the power supply to the halogen lamp 12, by comparing the sensed surface temperature of the fusing roller 11 with the set temperature and then turning on the halogen lamp 12 through the power switching part 19 when the detected temperature is below the set temperature and turning off the halogen lamp 12 when the detected temperature is over the set temperature, and thereby the surface temperature of the fusing roller 11 is maintained within a given range.
Also, the thermostat 16 functions as an overheating prevention means to protect the fusing roller 11 and its neighboring components, in case that the thermistor 15 and the controller 20 fail to regulate the surface temperature of the fusing roller 11.
However, the aforementioned fusing device is not without disadvantages. For example, in the conventional fusing device 10, since the halogen lamp 12 is turned on or off, regardless of a state or condition at each of the steps of the temperature control process for regulating the surface temperature of the fusing roller 11 in the print temperature, the electric power may not be efficiently supplied to the halogen lamp 12, thereby increasing power consumption and fluctuation in the surface temperature of the fusing roller 11 may be large.
When the fluctuation in the surface temperature of the fusing roller 11 is enlarged, the print temperature is unstably controlled, and thereby irregularly fusing the toner image onto the sheet of printing paper.